Conventionally, as disclosed in, for example, Patent Document 1, a so-called eccentrically-orbital gear device has been known, which includes an annular internal gear having internal teeth formed in the inner perimeter surface and an external gear provided inside the internal gear to produce eccentrically-orbital motion while being meshed with the internal gear, the external gear having external teeth which are smaller in number than the internal teeth. The input shaft of this gear device is rotatably supported by the main part of the device through a bearing. The input shaft is provided with an input-side eccentric part, which is inserted in a central hole formed in the central part of the external gear through a bearing such that it is rotatable around its center line. The main part of the device is provided with two crankshafts rotatably supported thereby. Each crankshaft is provided with a crank-side eccentric part which has the same eccentricity as that of the input-side eccentric part. This crank-side eccentric part is rotatably inserted in an insertion hole formed in the external gear at a radially outer position such that both of the crankshafts are engaged with the external gear.
In the gear device having such a configuration, rotation of the input shaft by a motor, or the like, drives the external gear due to the motion of the input-side eccentric part, so that the external gear causes the crankshafts to rotate around their center lines. Since the crankshafts are supported by the main part of the device and the crank-side eccentric parts are engaged with the external gear, the motion of the external gear is controlled by the crankshafts such that the external gear orbits while being inhibited from rotating and being meshed with the internal gear eccentrically from the center of the input shaft by the eccentricity of the input-side eccentric part. This eccentrically-orbital motion of the external gear decreases the rotation speed of the input shaft.
[Patent Document 1] Japanese Utility Model Publication for Opposition No. 2-45554